1. Field of the Invention
The present invention relates generally to rechargeable battery systems for electric vehicles, and more particularly, to fuel gauges for electric vehicles.
2. Description of the Prior Art
Vehicle operators and passengers alike are familiar with fuel gauges, giving a visual indication of fuel remaining.
With the move to electric vehicles, such fuel gauges have moved as well. In an electric vehicle, the fuel gauge displays an estimate of energy remaining in the vehicle's rechargeable battery.
One way in which this is done is by displaying on the fuel gauge an estimated charge state of the rechargeable battery system in the vehicle. The battery charge state is typically estimated using a known technique such as Coulomb counting. Coulomb counting counts the amount of energy going into and out of the battery, thereby maintaining an estimated charge state for the battery.
However, Coulomb counting has a problem with accumulated error which results in reduced accuracy over time. As understood in the art, Coulomb counting using discrete measurements is an approximation of an ideal process of integrating current flow continuously over time. Practical approaches use current measurements taken at discrete intervals of time to represent current flow over the interval. Such measurements taken during each interval represent an approximation, in which small errors accumulate over time. These small errors accumulate in the determined charge state over time. As the charge state maintained by Coulomb counting diverges from the actual charge state of the battery, so the reading on the fuel gauge becomes increasingly inaccurate over time. This creates a problem if the operator cannot rely on the fuel gauge to accurately indicate the battery charge state.
What is needed is a way to improve the accuracy of fuel gauges as may be used in battery operated devices, including electric vehicles.